One class of electrical devices is opto-electrical devices that use an organic material for light emission (or detection in the case of photovoltaic cells and the like). The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light (in light detecting devices this process essentially runs in reverse).
OLEDs can provide a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer, dendrimer or a light emitting low molecular weight material, and the other of which is a conductive polymer layer, for example a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
A number of different methods have been used to deposit the organic materials including vapour deposition and solution processing techniques such as spin-coating and inkjet printing.
FIG. 1 shows a vertical cross section through an example of an OLED device 100. In an active matrix display, part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminium, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired so that it does not obscure the display, by a conventional process of photolithography followed by etching.
An optional, substantially transparent hole transport layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapour deposition. The cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminium and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. In the case of a passive matrix device, mutual electrical isolation of cathode lines may be achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated. An encapsulant such as a glass sheet or a metal can is utilized to inhibit oxidation and moisture ingress.
In the case of a polymer-based OLED, layers 108 comprise a hole transport layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
FIG. 2 shows a view from above (that is, not through the substrate) of a portion of a three-colour active matrix pixellated OLED display 200 after deposition of one of the active colour layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, in the case of a passive matrix display the substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 are defined by banks 310, constructed around the perimeter of each well 308 and leaving an anode layer 306 exposed at the base of the well. The banks present a hydrophobic surface in order that they are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well. This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0989778. Alternatively, the plasma treatment step may be avoided by use of a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960. It will be appreciated that cathode undercut separators are not necessary in the case of an active matrix display, for example, in which case any given pixel is activated by means of a transistor associated with said pixel.
As previously mentioned, the bank and (where present) separator structures may be formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators; both these resists may be based upon polyimide and spin coated onto the substrate, or a fluorinated or fluorinated-like photoresist may be employed. In the example shown the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm square but the size of a pixel can vary considerably, depending upon the intended application.
The deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example: T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Sturm, “Ink-jet Printing of doped Polymers for Organic Light Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP O 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
Inkjet printing has many advantages for the deposition of materials for molecular electronic devices but there are also some drawbacks associated with the technique. It has been found that dissolved molecular electronic material deposited into a well with shallow edges dries to form a film with a relatively thin edge. FIGS. 4a and 4b illustrate this process.
FIG. 4a shows a simplified cross section 400 through a well 308 filled with dissolved material 402, and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. As can be seen a well is generally filled until it is brimming over. The solution 402 has a contact angle θc with the plasma treated bank material of typically between 30° and 40° for example around 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts, for example angle 402a in FIG. 4a. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate. The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a where it meets the face of a bank. In practice drying is complicated by other effects such as the coffee ring-effect. With this effect because the thickness of solution is less at the edge of a drop than in the centre, as the edge dries the concentration of dissolved material there increases. Because the edge tends to be pinned solution then flows from the centre of the drop towards the edge to reduce the concentration gradient. This effect can result in dissolved material tending to be deposited in a ring rather than uniformly. The physics of the interactions of a drying solution with a surface are extremely complicated and a complete theory still awaits development.
A further problem with inkjet deposition arises when filling wells which are large compared with the size of an inkjet droplet. A typical droplet from an inkjet print head has a diameter of approximately of 30 μm in flight and the droplet grows to approximately 100 μm in diameter when it lands and wets out. However it is difficult to produce drops of, say 100 μm in diameter (in flight) from a print head.
Filling a well or pixel of a similar size to a drop presents little problem as when the drop lands it spreads out and fills the well. This is illustrated in FIG. 5a which shows a well 500 for a long thin pixel of a type which is typically used in a RGB (red green blue) display. In the example of FIG. 5a the pixel has a width of 50 μm and a length of 150 μm with 20 μm wide banks (giving a 70 μm pixel pitch and a 210 μm full colour pitch). Such a well can be filled by three 50 μm droplets 502a, b, c as shown. Referring now to FIG. 5b this shows a well 510 for a pixel which is approximately four times larger than each dimension giving a pixel width of approximately 200 μm, more suitable for applications such as a colour television. As can be seen from the figure, many droplets 512 are needed to fill such a pixel. In practice, these tend to coalesce to form a larger droplet 514 which tends not to properly fill corners of the pixel (although FIGS. 5a and 5b and idealised and, in practice, the corners are not generally as sharp as they are shown). One way around this problem is to over fill the well to an extent that the dissolved material is pushed into the corners. This can be achieved by using a large number of dilute droplets and a high barrier around the well. Techniques for depositing large volumes of liquid are described in WO03/065474, which describes the use of very high barriers (for examples at page 8 lines 8 to 20) to allow the wells to hold a large volume of liquid without the liquid overflowing to adjacent wells. However such structures cannot easily be formed by photolithography and instead a plastic substrate is embossed or injection moulded. It is also desirable to be able to fill a well using fewer (higher concentration) droplets as this enables, inter alia faster printing.
Another problem associated with ink jet printing of organic opto-electrical devices such as those discussed above is that in the resultant device, the organic hole injecting layer can extend beyond the overlying organic semi-conductive layer providing a shorting path between the cathode and the anode at an edge of the well. By shorting path, we mean a path through which current flows in preference to flowing through the light emissive material. This problem is exacerbated if the contact angle of the conductive organic composition with the bank material is too low. This problem is further exacerbated if the conductivity of the organic hole injecting layer is too high.
One solution to the aforementioned problem is to modify the bank structure by, for example, providing a stepped bank structure which increases the length of the shorting path, thus increasing the resistance of the path resulting in less shorting. Such a solution has been proposed by Seiko Epson. However, providing a more complex bank structure is expensive and increases the complexity of the manufacturing method for the device.
Embodiments of the present invention seek to solve the aforementioned problems. In particular, embodiments of the present invention seek to solve the problems associated with containing solution processed electrically active material within wells defined by banks, ensure that the wells are completely wetted-out by the solution, and solve the problems associated with non-uniform film formation and shorting. Embodiments of the present invention seek to solve these problems while providing a simple and inexpensive manufacturing method.